Methods for preparing finished stainless steel coil include casting a stainless steel slab, hot-rolling the slab to reduce its thickness, cold rolling the steel to further reduce the thickness of the slab and finishing the steel. The hot-rolling steps and the cold-rolling steps are followed by annealing steps, and, if the steel is annealed in an oxidizing atmosphere, a subsequent descaling step to remove oxidation product (scale) on the surface of the coil.
A variety of methods have been devised to impart a satisfactory finish onto the stainless steel. After cold rolling the steel, the coil must be annealed. The steel can be annealed in air, resulting in formation of an oxide layer on the steel, which must be removed prior to finishing the steel. The oxide may be removed by pickling in an acid solution, which typically yields a dull finish. The steel can be temper rolled to improve the finish and workability of the product. If desired, the steel can be abraded (polished) to a desired finish, such as an American Iron and Steel Institute (AISI) #3 finish. The steel also can be buffed to a mirror finish.
A second method for annealing cold-rolled stainless steel is to bright anneal the steel. In the bright annealing process, the steel is annealed in a non-oxidizing atmosphere, such as a hydrogen and/or a hydrogen/nitrogen atmosphere, to preclude formation of surface oxides. The benefit of bright annealing is that no pickling step is necessary, and the bright-annealed surface is brighter than the surface of air-annealed and pickled steel. The difficulty with bright annealing stainless steel is that facilities for bright annealing are rare and expensive.
A process used commercially to brighten stainless steel is to buff the steel. Buffing typically consists of rubbing a slurry of water and rouge (very fine abrasive particles) against the steel surface with fabric brushes. Buffing consists of a two-stage process, including a relatively rough cutting step and a color step which utilizes a finer abrasive. The buffing process produces a mirror finish but is time consuming and extremely expensive, costing 5-10 times as much as coarse abrasive polishing used to produce the standard brushed finish which is designated as "#3 polish" by AISI.
A further method for preparing bright stainless steel is disclosed in U.S. Pat. No. 5,830,291. In that patent, a bright surface is prepared by annealing the cold-rolled steel in air, but at temperatures lower than those typically used for annealing cold-rolled steel. This results in a layer of scale which can be removed by a molten salt treatment followed by a mild acid pickling step. The finish of the resultant steel is bright and is suitable for embossing with a desired finish or texture.
A desired finish pattern can be embossed on the surface of the steel. To emboss the surface of the steel, conventional wisdom dictates that the steel must have a finish of the type obtained in a bright anneal. The embossing is carried out through the use of specially ground rollers which impart a desired finish on the steel. For example, a standard AISI #3 finish can be embossed on bright steel.
It is therefore an object of the current invention to obtain a suitable surface on an air annealed and acid-pickled stainless steel sheet, regardless of thickness or grade, without the need for a bright anneal or otherwise polishing the product prior to shipment.